Chilled water systems are commonly used to provide air conditioning in medium to large buildings. Such chilled water systems typically include multiple chillers disposed within the building at a central location, a plurality of heat exchangers disposed throughout the building at locations where air conditioning is to be accomplished, and a cooling tower disposed at a location that is exposed to the outside air. The chillers are coupled with the cooling tower by a condenser water loop, and with the heat exchangers by a chilled water loop. The basic components of each chiller are an evaporator that is coupled with the chilled water loop, a condenser that is coupled with the condenser water loop, and a compressor that intercouples the evaporator and condenser. Chilled water that is supplied to the chilled water loop by the evaporator absorbs building heat or "load" in the heat exchangers and is returned to the evaporator by the chilled water loop, with the absorbed heat being removed from the chilled water by the evaporator. The compressor converts an energy input, typically that of electrical energy, into a corresponding mechanical/thermal force that moves the absorbed heat from the evaporator to the condenser. The condenser transfers this absorbed heat to the condenser water which is circulated by the condenser water loop to and through the cooling tower, wherein the absorbed heat is transferred to the outside air, and which is then returned to the condenser. In many chilled water systems, the chillers are of the variable capacity type such as centrifugal, screw and absorption chillers whose operating capacity (measured in tons or Btu/hr) are varied by a throttle or other actuator that responds to a capacity adjustment signal. The capacity of centrifugal and screw chillers are controlled by regulating the refrigerant gas flow therein to maintain a desired chilled water temperature, and the capacity of absorption variable-capacity chillers are controlled by modulating the steam or hot water flow into a generator to maintain a desired chilled water temperature.
Chilled water systems are designed so that the maximum system capacity can accommodate worst-case conditions, that is, the highest outside air temperature, the highest humidity, and the largest building load to be encountered, taking into consideration also down-time of one or more chillers due to maintenance and equipment failure. As a result, the maximum system capacity is usually larger than that required on an average load day. Accordingly, the chilled water is typically operated at a capacity less than the maximum system capacity by selecting the chillers to be operated and by varying the capacity of the chillers so selected.
Typically, the selection of the chillers to be operated and adjustment of their capacity is done manually and is based on a number of factors such as building policy, the available operators, and their experience and equipment familiarity. It is highly impractical, however, for such manual control to achieve optimum efficiency of the chilled water system. Accordingly, the present invention is directed to a multiple chiller control method that optimizes the efficiency of a chilled water system by selecting the chillers to be operated and by varying the capacity of the chillers so selected without resort to manual control.